Measuring devices for the indirect or remote indication of the height of a body of liquid may be required when high pressure or other conditions preclude access to the liquid or the measuring location. Naturally, it is desirable to measure the height of a body of liquid directly, e.g. to read the level on the scale of a tank or the like. Frequently, however, especially in the case of high-pressure vessels in nuclear-reactor establishments, the pressure conditions under which the system is operated and/or some other parameters preclude accessibility and hence make necessary a remote indication of the height of a body of liquid in the receptacle.
Apart from mechanical, acoustical, optical and pneumatic processes for ascertaining the height of a body of liquid in an inaccessible vessel, it is known to provide an electrical measuring process in which the change of electrical capacity, ohmic resistance or the ratios between two resistances, may be determined.
For example, in German patent DT-PS No. 842 554, there is disclosed a measuring device or system which comprises three electrodes which are immersed in the liquid and two of which are connected in series. These electrodes provide a measured value in terms of the ratio of the contact or transfer resistance between the electrodes connected in series. In this conventional measuring device, the measured value has, with respect to the measured height of the liquid level, a sharp nonlinearity or deviation from a linear relationship between the measured value and the parameter which is to be determined.
It has been proposed, by modification of the geometric form of the electrode surfaces, to achieve a linearization of such measurements. However, such techniques are not satisfactory or even usable in all cases.
The German patent DT-PS No. 2 215 950 describes a process which has the goal of overcoming the nonlinearity of the system described in German patent DT-PS No. 842 554. This attempt to overcome the nonlinearity of the earlier system is, however, only successful when the measurement gives an absolute value which is independent of the supply voltage. The nonlinearity in the output signal or value even in these cases is, however, not fully eliminated.
German patent DT-PS No. 1 798 084 describes a device which enables the stray potential which develops on a longitudinally extending electrode immersed in the liquid and which is spaced by a relatively small distance from a resistance electrode traversed by a direct current. In this arrangement, however, it is disadvantageous that the stray current associated with the resistance electrode may only partly traverse the immersion electrode which serves as a potential sensor. The measurement and the height of the body of liquid can thus be in a nonlinear relationship.
In addition, with conventional devices, the system generally has a curved characteristic which results from a perturbation of the current flow lines at the surface of the liquid and from the leakage effects at the container bottom. The results thus have an unavoidable dependency on the ratio of the spacing to the length of the two electrodes. This system also operates with an applied direct current which gives rise to boundary conditions at the electrode/liquid interface with render the results unstable with time.
Still another device for measuring the height of liquid in a vessel is described in German published application (Auslegeschrift) DT-AS No. 1 908 921. In this system, two coaxially disposed electrodes are provided, one being composed of a low-ohmic metal and the other of a high-ohmic metal. The basic concept in this system is to make the resistance of one electrode so high that the liquid acts as a short circuit across it. In this prior-art system the disadvantage is observed that the precision is limited at best. It appears that the precision of the measurement is reduced whenever such devices are used because of an unavoidable wetting of portions of the high-ohmic electrode above the liquid level.